Embodiments described herein generally relate to trajectory control during drilling operations. More particularly, embodiments described herein relate to methods for determining drilling states of a downhole tool and controlling the trajectory thereof during the process of drilling a wellbore in a subterranean formation.
Directional drilling is the process of directing the downhole tool in a wellbore being drilled along a defined trajectory to a predetermined target. Trajectory control during drilling is the process of keeping the wellbore contained within some prescribed limits based on the measurement of the inclination and direction (azimuth) of the drill string at various formation depths utilizing various sensors. In drilling operations utilizing a rotary steerable system (RSS), a proportional controller may be used in trajectory control applications in order to implement the Inclination Hold (IH) mode and the Hold Inclination and Azimuth (HIA) mode; however, well-known dynamic conditions sometimes encountered during drilling, including, for example, axial vibration, lateral shock and vibration, torsional vibration, stick/slip, whirl, formation/bottom hole assembly tendency changes, bit walk tendencies, and dogleg output changes due to borehole overgauge, may reduce the effectiveness of the proportional controller (e.g., steady-state error). In addition, sensor response (measurement) delay and wellbore propagation delay, due to the distance between the drill bit and the sensor location, may also reduce the effectiveness of the proportional controller.
Accordingly, a proportional integral (PI) controller or any other controller with an integration term may be utilized to account for the dynamic conditions and the delays in the sensor and system responses. Although the use of the integrator in the PI controller may account for the above dynamic conditions and steady-state error, the integrator generally may not distinguish the drilling states (e.g., active drilling, circulating on bottom, back-reaming, etc.) from one another. In certain drilling states, e.g., back-reaming and circulating on bottom, the utilization of the integrator generally tends to inhibit trajectory control, thereby reducing accuracy of the wellbore trajectory during the directional drilling operations.